Satellite communication systems typically have employed large aperture antennas and high power transmitters for establishing an uplink to the satellite. Recently, however, very small aperture antenna ground terminals ("VSAT"), referred to as remote ground terminals, have been developed for data transmission at low rates. In such systems, the remote ground terminals are utilized for communicating via a satellite from a remote location to a central hub station. The central hub station communicates with multiple remote ground terminals, and has a significantly larger antenna, as well as a significantly larger power output capability than any of the remote ground terminals.
In a low-rate VSAT system, it is very important that good transmit frequency accuracy be maintained. A reference frequency of sufficient accuracy is prohibitively expensive, so ways are found of deriving a reference from the outroute bit timing. The outroute bit timing can be made very accurate at the hub, thus incurring the cost of a very accurate reference frequency only once per network, thereby amortizing its cost over many remote VSATs.
VSAT remote terminals obtain their reference frequencies from the bit timing of the outroute signal from the hub to the remote terminal. The local oscillator is usually compared with the outroute symbol rate to determine what frequency adjustment is necessary at the remote terminal. However, the local reference oscillator frequency has a tendency to drift and thus it is difficult to accurately adjust the ever-changing reference frequency to match the outroute symbol rate.
Conventional methods of maintaining frequency accuracy relied on analog bit timing recovery loops, multiple oscillators, and direct frequency counting of the local reference oscillator to obtain an accurate transmit frequency. These methods derive the reference frequency from the outroute bit rate over long time periods; thus, the local reference oscillator must remain stable throughout a long measurement period or the measurement will be invalid. Because of the amount of time required to accumulate enough counts to get an accurate measurement of reference frequency, the reference has to be ovenized so it would not drift significantly during the frequency measurement process. This type of conventional approach requires a costly high stability oscillator. An example of one type of conventional system is disclosed in U.S. Pat. No. 4,489,413 issued to Richmond et al.
None of the conventional frequency stabilization techniques provides the important advantages of quick and accurate measurement of reference frequency without using a costly reference oscillator thus reducing the short-term stability requirements of the local reference to that of a much less expensive temperature-compensated oscillator with thermal isolation.